Many cancer patients are treated using high energy X-Ray photon irradiation. For some cancers this presents the difficulty that the tumor might be located very close to radiation sensitive body parts. A recent development in ionizing radiation therapy is to produce a strongly position dependant intensity variation within a single beam by combining many intricate beam shapes which can, for example, be created by thin tungsten leaf collimators absorbing discreet regions of an x-ray beam. The leaf motion may either be in discrete steps or continuous. In a technique often called Intensity Modulated Radiation Therapy (IMRT), typically about seven such beams are combined from different static directions by means of a rotating gantry. Alternatively, both the leaf positions, and the beam direction, are continuously updated while the beam is on, a technique often referred to as Volumetric Arc Therapy (VMAT).
Before a patient is treated with such a complicated beam configuration, a pre-treatment measurement needs to be done. In this measurement, essentially, the treatment program is executed and the dose is measured using a water phantom. The need for the pre-treatment measurement doubles the time required to treat a patient.
It would therefore be desirable to measure the radiation dose in the patient during the radiation treatment itself, but this is very difficult. Measuring the dose downstream of the patient is notoriously difficult as the patient acts as a very complicated scattering centre.
U.S. Pat. No. 6,853,702 discloses a radiation dosimetry system, in which an imaging device is placed between the radiation source and the patient, and is used to form an image of the radiation field. EP-2108328-A discloses another system, in which a detector device, formed from an array of ionization chambers, is positioned between the multileaf collimator and the patient, and is used to determine the radiation dose delivered to the patient. Several millimeters of a water-equivalent material is typically placed upstream of the detector volume, in order for sufficient photons to interact with the detector to distinguish the therapeutic component of the beam from contaminant components such as electrons, which have differing radiation profile, and which only penetrate the skin of the patient.
However, these prior art systems fail to recognize that typically, if one puts a detector upstream of the patient, the beam is disturbed, resulting in photon scattering and the production of secondary electrons, and as a result a different dose in the patient is realized. In the case of a detector having a thickness of several millimeters of water-equivalent material, the beam might be attenuated by several percent, and the out of field intensity might increase by several tens of percent due to scattering.
According to a first aspect of the invention, there is provided a method of monitoring a radiation beam, the method comprising:
placing in the beam a MAPS detector;
exposing the MAPS detector to the radiation beam, such that photons of the radiation beam can interact with the MAPS detector;
determining a position of each identified interaction; and
estimating the radiation beam configuration from the determined positions of the interactions.
Thus, the detector of the present invention might interact with the radiation beam only 1% as much as the prior art detector. As a result, the disturbance to the beam is much smaller.